Death by fireblight

NC-140 dwarfing rootstock trial highlights fireblight sensitivities.

Apr 15, 2008

In what could be the only Golden Delicious planting in California, Dr. R. Scott Johnson said that the real value of the NC-140 rootstock trial at the University of California’s Kearney Agricultural Center is in the evaluation of fireblight susceptibility.

Since the NC-140 Golden Delicious dwarfing rootstock trial was planted in 2003, Johnson, who is the UC Cooperative Extension specialist for tree fruit, said that about one-third of the trees have been lost. "About ten percent of the trees randomly didn’t grow, but that wasn’t due to fireblight. But since the initial dying, most of the ones that die are due to fire­blight. Every year, we are pushing out a few more trees due to fireblight."

He notes that the Cornell-Geneva dwarfing series from New York has been the most interesting in the trial. None from Cornell have died from fireblight, though CG.30 was blown over by the wind during strong winds last winter, snapping off below the rootstock union. "But it ­didn’t die of fireblight."

The JM (Japan Morioka) rootstocks from Japan have shown some fireblight resistance, although one selection died from fireblight last year, he noted.

Johnson observed that there is a lot of variation in tree size in the NC-140 trial. Data from NC-140 trials are typically ­collected for ten years.

While many locations throughout the world successfully use M.9 as a dwarfing rootstock, Johnson said that M.9 doesn’t work as well in California. "It’s too hot, too dwarfing, and there’s too much sunburn. M.7 is also too weak for us."

Budagovsky 9 is also too weak for the area, he said, adding that it dies easily at the research center in Parlier.

Peach trial

The center’s NC-140 peach rootstock trial, planted in 2002, was removed last year because of the potential threat of plum pox. The Pumiselect rootstock selections planted in the trial came from Michigan State University’s breeding program at the Southwest Michigan Research and Extension Center in Benton Harbor.

"Even the slightest threat of plum pox causes us to tremble," Johnson said.

Several new rootstocks that were in the peach NC-140 trial looked promising, he said, explaining that the focus of the trial was on dwarfing rootstock for peaches.

Two new stone fruit rootstocks recently released by UC—Controller 5 and 9—are 50 to 90 percent the size of Nemaguard, the industry standard, but fruit size has been challenging, he said.

Krymsk 1 looked good in the trial, with Redhaven grown as the scion variety, but other varieties have not been as compatible on Krymsk, he said. More work on compatibility will be needed.

Hiawatha, another rootstock in the trial, showed inconsistent size. The Italian Penta had waterlogging problems, but Cadaman did well in high pH soils. A new NC-140 peach rootstock trial is scheduled to be planted in 2009.